Stressed out? A relaxing massage could help calm you down, but what if you're a fish? Turns out, the same thing applies.

Marta Soares of the ISPA University Institute in Lisbon, Portugal, and colleagues, noticed that in addition to removing parasites and dead skins, bluestreak cleaner wrasses often give their "clients" pelvic and pectoral fin massages - just think of them as the equivalent of human backrubs.

"We know that fish experience pain," Soares told NewScientist, "maybe fish have pleasure, too." To check whether those massages are beneficial, Soares put stressed out surgeonfish in two tanks with a model cleaner fish - one tank has a stationary model and the other a model that moves back and forth to provide physical stimulation. All surgeonfish approached the model, but only those in the tank with the moving model could get the physical fin massage.

When Soares and colleagues tested the level of the stress hormone cortisol of the two groups, the scientists found that only the surgeonfish that have contact with the moving model have lowered stress level.

It's obvious how cleaner wrasses benefit from the relationship - they get to feed on debris they remove from their clients, but what do the clients get in return? Why would the clients return to the wrasses and even "wait in line" to be cleaned?

"The discovery of a positive effect of physical contact in a reef fish ... resolves a long-standing paradox described in cleaning mutualism involving cleaner wrasses and their clients," said co-author Alexandra Grutter to Cosmos.